The use of correlation techniques in a data communications system decoding system to identify an incoming digital code word is well known. In a correlation decoding system a correlation detector is provided which matches an incoming digital code word on a bit-for-bit basis with a plurality of locally generated reference code words. When a substantial match with one of the reference code words is found, the correlator indicates this fact by an output signal, the amplitude of which represents the degree of bit-for-bit matching, and the reference code word with which the incoming digital code word is then being compared is identified as the received code word.
If a communication system employing a correlation detector in the decoding system uses Single Side Band (SSB) modulation for transmission and reception, Doppler shifts in the medium and carrier reinsertion errors in the receiver will shift each frequency in the demodulated output signal by some translation error which is the same for all output frequencies at the demodulator at any given instant, but which can vary with time due to changes in relative velocities of the transmitter and receiver and to variations in frequency standards in the transmitter and receiver. For SSB receivers and transmitters in the high frequency (HF) band (2 to 30 MHz), frequency translation errors in the demodulator output of up to .+-.100 Hz are common.
When a frequency translation error occurs, it has been found that the output signal of the correlation detector rotates in phase at the translation error frequency providing the envelope of the output signal of the correlation detector with an amplitude which is sinusoidal, varying at the translation frequency. With very low (or even zero) translation errors, if the phase of the correlation detector output is near a sinusoidal null of the amplitude variation, the correlation detector output will be low or zero and will remain so until the phase rotates away from the null. Such a low output of the correlation detector may cause the decoding system to fail to properly recognize a match between an incoming digital code word and one of the internally generated reference code words.
It is also often desirable to retrofit a data communications system to an existing operational radio transmitter and receiver system to avoid the expense of designing an entirely new transmission and reception system for data transmission. If a data communications system is retrofit to High-Frequency (HF) radio transmitters and receivers designed for Single Side Band (SSB) voice transmissions, a problem exists in that the limited audio band pass (usually 300 to 2500 Hz) of the transmitters and receivers prohibits use of wide-band spread-spectrum techniques commonly employed at UHF and microwave frequencies to improve code transmission reliability. At the same time the many forms of signal degradation associated with HF transmitters and receivers, such as fading, high interference, multipath propogation, etc. places demands on the data communications system which are not easily satisfied by simple frequency-shift keying or other conventional digital transmission techniques. To successfully permit utilization of in place High-Frequency SSB radio transmitters and receivers for data communications, the data communications system must:
a. provide a unique wave form in space which is highly unlikely to be duplicated by probable sources of interference or to be modified by such interference to produce undetected output errors;
b. provide a high degree of error detection and correction;
c. provide the ability to accept the waveform to which a receiver is synchronized while rejecting interference (either from other transmitters or from multipath propagation of the desired transmission) from similar waveforms not precisely in synchronism;
d. permit as high a communications data rate as possible within the transmitter/receiver bandpass limitations;
e. be implemented through addition of external encoder and decoder circuitry without modification of existing transmitters and/or receivers and without access to transmitter carrier frequency control circuits to achieve phase or frequency modulation nor to receiver RF or IF circuitry as would be required for demodulation of frequency or phase modulation or conventional correction of SSB translation errors; and
f. provide for synchronization of the demodulator to received signals and compensation for frequency translation errors over the same range of signal-to-interference ratios as required for signal processing after synchronization is achieved.